[PATCH] drm/prime: Only call dma_map_sgtable() for devices with DMA support

[Thomas Zimmermann]

Hi

Am 19.02.21 um 16:53 schrieb Alan Stern:
> On Fri, Feb 19, 2021 at 02:45:54PM +0100, Christian König wrote:
>> Well as far as I can see this is a relative clear NAK.
>>
>> When a device can't do DMA and has no DMA mask then why it is requesting an
>> sg-table in the first place?
> 
> This may not be important for your discussion, but I'd like to give an
> answer to the question -- at least, for the case of USB.
> 
> A USB device cannot do DMA and has no DMA mask.  Nevertheless, if you
> want to send large amounts of bulk data to/from a USB device then using
> an SG table is often a good way to do it.  The reason is simple: All
> communication with a USB device has to go through a USB host controller,
> and many (though not all) host controllers _can_ do DMA and _do_ have a
> DMA mask.

It's semi-related. One idea we had for speeding up transfers is to use 
the USB controller's DMA functionality and allocate framebuffers 
accordingly. It needs a bit of work in the DRM memory-management code 
IIRC. And we do some internal modifications if the frambuffer's data, so 
direct forwarding from renderer to USB controller is not always possible.

Best regards
Thomas

> 
> The USB mass-storage and uas drivers in particular make heavy use of
> this mechanism.
> 
> Alan Stern
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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